Methods and extraction units employing vapor draw compositional analysis

ABSTRACT

Embodiments of extraction unit and an analysis method are provided. In one embodiment, the analysis method includes the steps of providing a feed stream and a species-selective solvent to the distillation column, drawing a vapor sample from the distillation column, condensing the vapor sample, and analyzing at least a portion of the condensed vapor sample.

FIELD OF THE INVENTION

The present invention relates generally to extraction and, moreparticularly, to embodiments of an extraction unit and method employinga vapor draw to provide compositional analysis of material streams withreduced time lag.

DESCRIPTION OF RELATED ART

Extraction units are commonly utilized within various processingindustries to separate feed streams into raffinate and extract streamsfor petrochemical, gasoline, and other applications. In thepetro-chemical industry, specifically, extractive distillation andconventional extraction units are often utilized to separate hydrocarbonfeed stocks into raffinate streams consisting primarily of non-aromatichydrocarbons and extract streams consisting primarily of aromatichydrocarbons, such as benzene, toluene, xylene, and other aromaticspecies. In one common extractive distillation process, a hydrocarbonfeed stream is introduced into an intermediate section of an ExtractiveDistillation (“ED”) column containing a series of stacked trays. A polarsolvent, such as sulfolane, is continually circulated through the EDcolumn, often in the presence of steam. The polar solvent contacts thefeed stream within the ED column trays and predominately interacts withthe hydrocarbon components having stronger polarities when interactingwith the solvent to decrease the volatility of the aromatic extractrelative to the non-aromatic raffinate. The non-aromatic raffinate iscollected from the ED column via an overhead outlet while the extract,solvent, and water, if present, exits the ED column through a loweroutlet as a rich solvent stream. The rich solvent stream is thendirected into a second distillation column (commonly referred to as a“recovery column” or a “solvent recovery column”) to separate theextract from the solvent and thereby provide a highly pure extractstream of the desired aromatic product. Commonly, in the conventionalextraction process, the feed and solvent are first contacted in aliquid-liquid scheme and then are introduced as a single stream to theED column (referred to as a “stripper” or “extractive distillationstripper” column).

On-line analyzers are commonly utilized within extraction units of thetype described above to ensure satisfaction of purity standards. Certainextractive distillation and conventional extraction units also employ anon-line analyzer to provide analytical information regarding thecomposition of liquid samples taken from a location downstream of the EDcolumn. The liquid sample is taken from a location downstream of the EDcolumn (e.g., from the receiver of a recovery column) to minimize theliquid sample's solvent content, which, if undesirably high, can promotephase separation and dilute the liquid sample thus rendering detectionof trace levels of contaminants more difficult. The on-line analyzermeasures the presence of at least one component or contaminant withinthe liquid sample to determine sample purity. For example, the analyzermay measure the presence of non-aromatics or other contaminants in theextract aromatic stream. The analytical data provided by the analyzermay then be utilized to adjust one or more operational parameters of theextraction unit, if needed, to maintain extract purity within a desiredrange.

Conventional on-line analyzer systems of the type described aboveprovide a relatively straightforward and accurate compositional analysisof liquid samples. However, due to the downstream location from whichthe liquid samples are taken, compositional changes taking place withinthe ED column due to, for example, process parameter adjustments are notfully detectable until well after such changes first occur andequilibrium is reached within the downstream equipment from which theliquid samples are taken. A significant time lag thus occurs between thecurrent material conditions within the ED column and the analytical dataprovided by conventional on-line analyzer systems, which introducesundesirable uncertainties into process control.

There thus exists an ongoing demand to provide embodiments of anextraction unit and analysis method wherein analysis data is providedwith a significant reduction in time lag as compared to conventionalextraction units and analysis methods of the type described above. Otherdesirable features and characteristics of embodiments of the presentinvention will become apparent from the subsequent Detailed Descriptionand the appended Claims, taken in conjunction with the accompanyingDrawings and the foregoing Description of Related Art.

SUMMARY OF THE INVENTION

Embodiments of an analysis method are provided for use in conjunctionwith an extraction unit including a distillation column. In oneembodiment, the analysis method includes the steps of providing a feedstream and a species-selective solvent to the distillation column,exposing a feed stream to a species-selective solvent, drawing a vaporsample from the distillation column, condensing the vapor sample, andanalyzing at least a portion of the condensed vapor sample.

Embodiments of an extraction unit are further provided for separating afeed stream utilizing a species-selective solvent. In one embodiment,the extraction unit includes a distillation column that receives thefeed stream and species-selective solvent, whether as a single stream orseparate streams; a vapor draw fluidly coupled to the distillationcolumn and configured to draw vapor samples therefrom during operationof the extraction unit; and an analyzer system fluidly coupled to thevapor draw and configured to measure the presence of at least onecomponent within the vapor samples. In certain embodiments, the vaporsample may be separated into at least two liquid phases prior toanalysis, and analysis is performed on at least one phase separated fromthe condensed vapor sample.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will hereinafter be described inconjunction with the following drawing figures, wherein like numeralsdenote like elements, and wherein:

FIG. 1 is a schematic diagram of an extraction unit including a vapordraw analyzer system in accordance with an exemplary embodiment;

FIG. 2 is a schematic diagram illustrating an exemplary vapor drawanalyzer system suitable for use as the analyzer system shown in FIG. 1;and

FIG. 3 is a flowchart illustrating an exemplary vapor draw analysismethod that can be carried-out by the vapor draw analyzer system shownin FIG. 2.

DETAILED DESCRIPTION

The following Detailed Description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by any theorypresented in the preceding Description of Related Art or the followingDetailed Description.

Various embodiments of the vapor draw analysis method and extractionunit contemplated herein provide analysis data with significantreduction in time lag as compared to conventional extraction units andanalysis methods. In contrast to conventional analysis methods whereinliquid samples are taken from a location downstream of a distillationcolumn (e.g., from a recovery column receiver), embodiments of theanalysis method and extraction unit described below providecompositional profile data reflective of current or near currentmaterial conditions within a distillation column by drawing vaporsamples directly therefrom. This results in a significant reduction indata time lag, which enables the extraction to be controlled in a moreprecise manner. Further, by employing a vapor draw to remove vaporsamples from a distillation column, the amount of solvent present in agiven vapor sample is greatly reduced, which minimizes dilution of thevapor sample and which reduces the likelihood of phase separation withinthe vapor sample. In certain embodiments, the amount of solvent in theanalyzed portion of the vapor sample is still further reduced byseparating the condensed vapor sample into at least two liquid phases,one of which contains little to no water or solvent, and analyzing theliquid phase containing little to no water or solvent, as furtherdescribed below.

While primarily described below in the conjunction with an aromaticseparation process, embodiments of the vapor draw analysis methodcontemplated herein can also be utilized in conjunction with other typesof separation processes, including processes utilized to separateolefins from non-olefins and sulfur-containing species fromnon-sulfur-containing species. Furthermore, although described below inconjunction with a particular type of extractive distillation unit,embodiments of the vapor draw analysis method are by no means limited tousage in conjunction with a particular type of extractive distillationunit and may be performed in conjunction with various other types ofextraction units, such as conventional extraction units. Embodiments ofthe vapor draw analysis method are also amenable to performance inconjunction with Udex and Carom separation processes commerciallyimplemented by UOP, LLC (formerly “Universal Oil Products”),headquartered in Des Plaines, Ill.

FIG. 1 is a schematic illustrating an extraction unit 10 including avapor draw analyzer system 12 illustrated in accordance with anexemplary embodiment and suitable for performing embodiments of thevapor draw analysis method described below. In this particular example,extraction unit 10 assumes the form of an extractive distillation unitincluding two distillation columns, namely, an extractive distillation(“ED”) column 14 and a recovery column 16 (also commonly referred to asa “solvent recovery column”). The various components of extraction unit10 are described in detail below to provide an exemplary context inwhich vapor draw analyzer system 12 and embodiments of the vapor drawmethod can be understood. Alternative embodiments of extraction unit 10may, however, assume other forms (e.g., that of a conventionalliquid-liquid extraction unit) and may include components other thanthose included in the exemplary embodiment described below.

With continued reference to FIG. 1, ED column 14 assumes the form of avertically-oriented vessel containing a series of stacked trays; e.g.,in one embodiment, ED column 14 contains between 50 and 90 real trays.During operation of extraction unit 10, a feed stream 18 is directedinto a feed port 19 provided in, for example, an intermediate or middlesection of ED column 14. In embodiments wherein extraction unit 10 isutilized to carry-out aromatic separation, feed stream 18 containsaromatic and non-aromatic hydrocarbon components. As appearing herein,the terms “aromatic,” “aromatic hydrocarbon,” and the like are utilizedto denote a hydrocarbon containing one or more rings of unsaturatedcyclic carbon radicals where one or more of the carbon radicals can bereplaced by one or more non-carbon radicals. An exemplary aromaticcompound is benzene having a C₆ ring containing three double bonds. Feedstream 18 may also contain other hydrocarbon molecules (e.g.,straight-chain, branched, or cyclic alkanes, alkenes, alkadienes,alkynes, and alkenylbenzenes), various impurities (e.g., hydrogen,metals, sulfur, etc.), and/or water. In many cases, feed stream 18 willcomprise a relatively wide boiling range mixture of benzene, toluene,and xylenes admixed with corresponding boiling range paraffins andnaphthenes. The source from which feed stream 18 is derived will varyamongst applications. However, as a first example, feed stream 18 may bedebutanized or depentanized effluent produced utilizing a conventionalcatalytic reforming unit. Alternatively, as a second example, feedstream 18 may be a liquid byproduct from a hydrocarbon cracking unithydrotreated to saturate olefins and diolefins and to remove tracelevels of sulfur and nitrogen contained therein.

During operation of extraction unit 10, a species-selective solventstream 20 is introduced into ED column 14 via a solvent inlet port 21,which is conveniently positioned above the feed stream inlet.Aromatic-selective solvent stream 20 can contain any compound or classof compounds that interacts with the components of feed stream 18 toalter their relative volatility and thereby enable the separationthereof. In embodiments wherein extraction unit 10 is utilized inaromatic separation, solvent stream 20 will comprise anaromatic-selective solvent, which is at least partially polar andexhibits a greater affinity for the more aromatic hydrocarbons. Morespecifically, solvent stream 20 may contain a solvent compound includinga five-member ring containing one atom of sulfur and four atoms ofcarbon with two oxygen atoms bonded to the sulfur atom of the ring. In apreferred embodiment, the solvent comprises 1,1-dioxidetetrahydrothiofuran or tetrahydrothiophene 1,1-dioxide (also known as“tetramethylene sulfone” and commonly referred to as “sulfolane”). Anon-exhaustive list of additional exemplary solvent compounds that maybe included in solvent stream 20 includes 2-sulfolene, 3-sulfolene,2-methylsulfolane, 2-4-dimethyl sulfolane, methyl-2-sulfonylether,N-aryl-3-sulfonylamine, ethyl-3-sulfonyl sulfide, 2-sulfonylacetate,diethyleneglycol, polyethyleneglycol, dipropyleneglycol,polypropyleneglycol, dimethylsulfoxide, N-methylpyrrolidone,glycol-amine, glycols, and glycol ethers includingpolyethyleneglycolether, N-methyl-2-pyrrolidone, and N-formylmorpholine. The solvent-to-feed volume ratio may be about 1:1 to about20:1 depending on operating conditions within ED column 14, thecomposition of feed stream 18, and other such factors.

As solvent stream 20 flows over the trays included within ED column 14,solvent stream 20 contacts feed stream 18. Solvent stream 20 interactsprimarily with the aromatic components included within feed stream 18thereby reducing their volatility relative to the non-aromaticcomponents included within stream 18. The components having the greatestvolatilities after exposure to solvent stream 20 exit ED column 14predominately via an overhead outlet 23 as light stream 22; whilepredominately the components having lower volatilities after exposure tosolvent stream 20, along with the solvent, exit ED column 14 via a loweroutlet 25 as a solvent rich stream 24 (also commonly referred to as a“lower stream” or a “bottoms stream”). In embodiments wherein ED column14 is utilized in aromatic separation, light stream 22 will typicallycontain substantially all (e.g., conveniently at least 99 wt %,preferably at least 99.5 wt %, and more preferably at least 99.9 wt %)of the non-aromatic components introduced into ED column 14.Additionally, light stream 22 will often contain trace amounts ofaromatics, solvent, and entrained water droplets, if water is introducedinto ED column 14 during processing. By comparison, rich solvent stream24 will typically contain substantially all (e.g., conveniently at least90 wt %, preferably at least 95 wt %, and more preferably at least 99 wt%) of the aromatic hydrocarbons and aromatic-selective solvent. Richsolvent stream 24 may also include certain non-aromatic components,contaminants, and water.

Water is preferably, although not necessarily, directed into ED column14 during operation of extraction unit 10 as a portion of the solventstream 20 shown in FIG. 1. The steam condensing in the overhead systemmay also be collected and returned to ED column 14 (e.g., via a pump 27and a water return conduit 29, as described below) to ensure that thenet flow of water leaving column 14 is balanced by the majority of thewater introduced into column through stream 20. Additionally vapor maybe caused to rise through ED column 14 by a reboiler circuit, such asreboiler circuit 26 shown in FIG. 1. As will be described more fullybelow, the operational parameters of reboiler circuit 26, such as theheat duty of at least one reboiler 28 included within reboiler circuit26, can be adjusted during the extraction process to control componentseparation and thereby maintain extract or raffinate purity withinpredetermined standards. By way of non-limiting example, the operatingparameters of extraction unit 10 may be controlled to maintain thepressure within ED column 14 between approximately 12 kilopascal (kPa)and 380 kPa, the overhead temperature within column 14 within a range ofapproximately 50° C. and 70° C., and the bottoms temperature withincolumn 14 within a range of approximately 70° C. and 260° C. during theextraction process. In embodiments wherein ED column 14 is utilized foraromatic separation and solvent stream 20 comprises sulfolane, reboilercircuit 26 is preferably controlled to maintain the bottoms temperaturebetween approximately 150° C. and 200° C.

After exiting the overhead outlet of ED column 14, light stream 22 isreceived and condensed by a condenser 30, such as an air-cooled heatexchanger (illustrated) or a liquid-cooled heat exchanger, or both.Light stream 22 is then collected, in its condensate form, within areceiver 32, which is typically horizontally-oriented. A pump 34 isfluidly coupled to an outlet of receiver 32 and, when energized,circulates a portion of the condensed light stream back to an uppersection of ED column 14 as a reflux stream 36. A portion of thecondensed vapor is water which is preferably recycled to the ED columnvia the separate inlet nozzle located below feed stream inlet 19, asindicated in FIG. 1 at 29. The remainder of condensed light stream iswithdrawn from extraction unit 10 as a raffinate stream 38. Inembodiments wherein ED column 14 is utilized in aromatic separation,raffinate stream 38 will consist primarily of nonaromatic hydrocarbons,but may also contain a certain weight percentage of aromatichydrocarbons. In particular, raffinate stream 38 conveniently includesup to 10 wt % aromatics, preferably contains less than 1 wt % aromatics,and more preferably contain less than 0.1 wt % aromatics. Raffinatestream 38 may also contain varying amounts of water depending upon theamount of water entrained in light stream 22 from which raffinate stream38 is derived, whether light stream 22 is separated, and, if so, theconditions under which light stream 22 was separated. As previouslyindicated, a second pump 27 may also be fluidly coupled to a secondoutlet of condenser 30 to return water collected within condenser 30 toED column 14, as shown in FIG. 1 at 29.

After leaving ED column 14, rich solvent stream 24 is directed intorecovery column 16 to separate the extract component or components fromthe solvent. As was the case previously, steam may be introduced intorecovery column 16 by, for example, a reboiler circuit 40 to promotecomponent separation at lower temperatures and to help minimizedegradation of the solvent. Recovery column 16 thus produces a secondoverhead or light stream 42, which is recovered through an overheadoutlet 43; and a lean solvent stream 44, which is recovered through alower outlet 45 and ultimately recycled back to ED column 14 withsolvent stream 20 in the manner shown in FIG. 1. In embodiments whereinextraction unit 10 is utilized in aromatic separation, light stream 42contains the desired aromatic product or products, such as benzene,toluene, and/or xylene. Lean solvent stream 44, by comparison, containsthe solvent, water (if present), and relatively minor amounts ofcontaminants, such as aromatic hydrocarbons, non-aromatic hydrocarbons,and/or other components including organic compounds having a higherboiling point or a lower relative volatility than light stream 42.

After exiting recovery column 16, light stream 42 is condensed in muchthe same manner as is light stream 22 produced by ED column 14. That is,light stream 42 may be condensed by an overhead condenser 46, such as anair-cooled heat exchanger (illustrated) or a liquid-cooled heatexchanger, or both. A receiver 48 collects the condensed light streamfrom overhead condenser 46; and a pump 50, which is fluidly coupled toan outlet of receiver 48, draws the condensed light stream from receiverand returns a portion of the condensed light stream to an upper sectionof recovery column 16 as reflux 52. The remainder of the condensed lightstream is withdrawn from extraction unit 10 as a final extract stream54. In embodiments wherein ED column 14 is utilized in aromaticseparation, extract stream 54 will contain the desired aromatic productand possibly water, which may be later separated from the aromaticproduct in a conventionally-known manner (e.g., utilizing a separatevessel having a water boot). A second pump 57 may also be fluidlycoupled to a second outlet of receiver 48 to return water collectedwithin receiver 48 to recovery column 16 proximate reboiler 40, asindicated in FIG. 1 at 59.

As noted above, and in contrast to conventional extraction units,extraction unit 10 further includes a vapor draw analyzer system 12.During operation of extraction unit 10, analyzer system 12 continuallyremoves vapor samples from ED column 14 through a vapor draw(generically represented in FIG. 1 by line 56). By removing vaporsamples from ED column 14 directly as opposed to a location downstreamof column 14, analyzer system 12 is able to provide analytical datadescribing current or near current material conditions within ED column14 with relatively little time lag. Furthermore, as thespecies-selective solvent within ED column 14 is primarily in a liquidphase, relatively little solvent will be present in a given vapor samplethereby avoiding the difficulties associated with high solvent contents(e.g., excessive phase separation and sample dilution). The amount ofsolvent in the analyzed portion of a given vapor sample may stillfurther be reduced in certain embodiments by separating the vapor sampleinto at least two liquid phases, one of which contains little to nosolvent, and analyzing the liquid phase containing the lesser solvent.The particular manner in which a given vapor sample may be processed andanalyzed by vapor draw analyzer system 12 is described more fully belowin conjunction with FIGS. 2 and 3.

Vapor draw 56 is preferably fluidly coupled to ED column 14 atapproximately the location where the concentration of the component (orcomponents) to be measured by analyzer system 12 is the highest. If, forexample, it is desired to measure a quantity indicative of impuritieswithin raffinate stream 38, the vapor draw may be fluidly coupled to EDcolumn 14 between the feed stream inlet and the light stream overheadoutlet, such as a few trays below the lean solvent inlet. Conversely, ifit is desired to measure a quantity indicative of impurities within richsolvent stream 24, vapor draw 56 may be fluidly coupled to ED column 14between the feed stream inlet and the lower outlet through which richsolvent stream 24 flows, as generally shown in FIG. 1. In this lattercase, vapor draw 56 is preferably fluidly coupled to ED column 14between the feed tray and the bottommost tray (generically representedin FIG. 1 by dashed boxes 57 and 59, respectively) and, more preferably,fluidly coupled to ED column 14 within the range of the bottom tentrays. In embodiments wherein extraction unit 10 is utilized in aromaticseparation, vapor draw 56 is preferably positioned at a locationsufficient to ensure that the total non-aromatic components includedwithin the vapor sample (or, more generally stated, “impurities ofinterest”) does not exceed about 1 wt % and, more preferably, that thetotal non-aromatics within the vapor sample is less than about 0.5 wt %.

In a preferred embodiment, the analytical data generated by analyzersystem 12 is utilized to adjust at least one operation parameter ofextraction unit 10, such as the heat output of reboiler 28. Adjustmentsto the operational parameters of extraction unit 10 based uponanalytical data generated by analyzer system 12 may be carried-outpursuant to either an open loop or a closed loop control scheme. Forexample, the data generated by analyzer system 12 may be utilized todetermine appropriate parameter adjustments that are manuallyimplemented. Alternatively, analyzer system 12 may be configured toautomatically adjust at least one operational parameter of extractionunit 10 as a function of the measured quantities of a component (orcomponents) of interest. More specifically, and as indicated in FIG. 1by dashed line 58, analyzer system 12 may be operably coupled toreboiler 28 and configured to automatically adjust the heat duty ofreboiler 28 as a function of the analytical data generated by analyzersystem 12. Additional operational parameters of extraction unit 10 thatmay be adjusted in response to generated analytical data include, butare not limited to, ED column reflux, ED column pressure, solvent streaminlet temperature, feed stream inlet temperature, and the like. Aftersample analysis, the condensed vapor sample may be returned to ED column14 via a return conduit 60 or, instead, simply purged.

FIG. 2 is a schematic diagram illustrating vapor draw analyzer system 12in greater detail in accordance with one exemplary implementation. Inthis particular example, analyzer system 12 includes a condenser 62,which is fluidly coupled to an outlet of vapor draw 56; a vertical tankor receiver/phase separator 64, which is fluidly coupled to an outlet ofcondenser 62; a pump 66, which is fluidly coupled to an outlet ofreceiver/phase separator 64; at least one analyzer 70, which is fluidlycoupled to an outlet of pump 66; and at least one controller 72, whichis operably coupled to an output of analyzer 70. In embodiments whereinvapor draw analyzer system 12 is configured to automatically adjust theheat duty of reboiler 28, an output of controller 72 may also beoperably coupled to a reboiler control valve 82 regulating the flow ofheated water to reboiler 28, as indicated in FIG. 2 by control line 80.Analyzer 70 may be fluidly coupled to pump 66 by way of a flow loop 74including a bypass flow loop through valve 76. During operation ofanalyzer system 12, flow control valve 76 can be adjusted, asappropriate, to control the flow rate to analyzer 70 to ensure that thecapacity of analyzer 70 is not exceeded regardless the flow output ofpump 66. As further indicated in FIG. 2 at 60, an outlet ofreceiver/phase separator 64 and an outlet of analyzer 70 may each befluidly coupled to a sample return inlet fluidly coupled to ED column 14at, for example, a location immediately below vapor draw 56. If desired,a flow indicator 78 may be positioned downstream of flow control loop 74to monitor the flow rate of the vapor samples returned to ED column 14.

Many of the components included within vapor draw analyzer system 12 areconventionally known and will consequently not be described in greaterdetail other than to note the following. Condenser 62 may comprise anydevice or assemblage of devices suitable for condensing a substantialportion of the vapor samples received through vapor draw 56 by reducingvapor sample temperature or increasing vapor sample pressure. In theillustrated exemplary embodiment, specifically, condenser 62 assumes theform of a liquid-cooled (e.g., water-cooled) heat exchanger.Receiver/phase separator 64 may comprise any device or devices suitablefor separating a condensed vapor samples into at least two liquidphases. In a preferred embodiment, receiver/phase separator 64 assumesthe form of or includes either a decanter-type phase separator or acyclone-type separator (also commonly referred to as a “centrifugalseparator”). Analyzer 70 may comprise any instrument or instrumentssuitable for measuring, either directly or indirectly, the presence ofat least one component or contaminant in a selected liquid phase of thecondensed vapor sample. For example, analyzer 70 may comprise one ormore gas chromatograph, near-infrared spectroscopic, and Fouriertransform infrared spectroscopic instruments. In a preferred embodimentwherein analyzer 70 is utilized to detect non-aromatic components in anaromatic extract stream or aromatic components in a non-aromaticraffinate stream, analyzer 70 comprises a gas chromatograph. Analyzer 70may also include a non-illustrated controller. Finally, controller 72may comprise, or be associated with, any suitable number of individualmicroprocessors, memories, power supplies, storage devices, interfacecards, and other standard components known in the art. Controller 72 mayalso include or cooperate with any number of software programs orinstructions designed to carry-out the various methods, process tasks,calculations, and control functions set-forth herein.

FIG. 3 is a flowchart illustrating an exemplary vapor draw analysismethod 90 that may be carried-out by exemplary vapor draw analyzersystem 12. Referring collectively to FIGS. 2 and 3, analysis method 90commences with the removal of at least one vapor sample from ED column14 through vapor draw 56 (STEP 92, FIG. 3). Next, at STEP 94 (FIG. 3),the vapor sample is condensed within condenser 62. Receiver/phaseseparator 64 then receives the condensed vapor sample from condenser 62and separates the sample into at least two liquid phases (STEP 96, FIG.3). A selected phase (or phases) of the condensed vapor sample issubsequently drawn from receiver/phase separator 64 by pump 66 andsupplied to analyzer 70 for compositional analysis (STEP 98, FIG. 3). Inparticular, during STEP 98, analyzer 70 may measure the selected liquidphase to determine the quantity of at least one component of interest,such as the quantity of one or more components indicative of raffinateimpurities in an extract stream or indicative of extract impurities in araffinate stream. The analytical data generated by analyzer 70 duringSTEP 98 is provided to controller 72, which then determines ifadjustments to the operational parameters of extraction unit 10 arerequired to maintain the extract or raffinate streams withinpredetermined purity standards. If determining that operationaladjustments are required, controller 72 implements these adjustments(STEP 100, FIG. 3). For example, if determining that the heat duty ofreboiler 28 should be increased or decreased, controller may command 80reboiler control valve 82 to a more open or closed position. Finally, atSTEP 102 (FIG. 3), the condensed vapor sample is returned to ED column14 through vapor sample return conduit 60. After returning the condensedvapor sample to ED column 14, analyzer system 12 may collect a newsample and repeat vapor draw analysis method 90, as indicated in FIG. 3at 104 and 106, to provide continual and rapid compositional analysis ofthe material stream of interest.

The foregoing has thus provided exemplary embodiments of analysis methodand extraction unit wherein compositional profile data is obtained in ahighly efficient manner and, in certain embodiments, utilized to adjustone or more operational parameters of the extraction unit. In contrastto conventional analysis methods wherein liquid samples are taken from alocation downstream of a distillation column (e.g., from a recoverycolumn receiver), the above-described analysis method providescompositional profile data reflective of current or near currentmaterial conditions within a distillation column (e.g., ED column 14) bydrawing vapor samples directly therefrom. This results in a significantreduction in data time lag, which enables the extraction to becontrolled in a more precise manner. Further, by employing a vapor drawto remove vapor samples from a distillation column, the amount ofsolvent present in a given vapor sample is greatly reduced, whichminimizes dilution of the vapor sample and which reduces the likelihoodof phase separation within the vapor sample. In certain embodiments, theamount of solvent in the analyzed portion of the vapor sample is stillfurther reduced by separating the condensed vapor sample into at leasttwo liquid phases, one of which contains little to no solvent, andanalyzing the liquid phase containing little to no solvent as previouslydescribed.

While the exemplary analysis method was described above in conjunctionwith a particular type of extractive distillation unit, embodiments ofthe analysis method are equally applicable to other types of extractionunits, including conventional liquid-liquid extraction units. Notably,in the case of a liquid-liquid extraction unit, the feed stream andspecies-selective solvent are contacted upstream of the distillationcolumn and then subsequently provided to or received by the distillationcolumn as a single stream. Thus, in both extraction distillation andliquid-liquid extraction techniques, a feed stream and aspecies-selective solvent are provided to a distillation column aseither a single stream (in the case of liquid-liquid extraction) or asseparate streams (in the case of extractive distillation). The locationof the vapor draw will, of course, vary depending upon extraction unittype and the particular component or components measured duringanalysis. For example, in the case of a conventional extraction unitincluding a stripper column, the vapor draw is conveniently taken fromthe stripper column's bottom section. While primarily described above inthe exemplary context of aromatic separation, embodiments of the vapordraw analysis method and vapor draw analysis system are by no meanslimited usage in conjunction with aromatic separation processes and canalso be utilized in conjunction with processes utilized to separateolefins from non-olefins and sulfur-containing species fromnon-sulfur-containing species.

While at least one exemplary embodiment has been presented in theforegoing Detailed Description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing Detailed Description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention, it beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set forth in the appendedClaims and their legal equivalents.

1. An analysis method for use in conjunction with an extraction unitincluding a distillation column, the analysis method comprising:providing a feed stream and a species-selective solvent to thedistillation column; drawing a vapor sample from the distillationcolumn; condensing the vapor sample; and analyzing at least a portion ofthe condensed vapor sample.
 2. An analysis method according to claim 1further comprising separating the condensed vapor sample into at leasttwo phases, and wherein the step of analyzing comprises analyzing atleast one phase separated from the condensed vapor sample.
 3. Ananalysis method according to claim 1 further comprising the step ofcontrolling at least one operational parameter of the extraction unit inresponse to analytical data produced during the step of analyzing.
 4. Ananalysis method according to claim 3 wherein the extraction unit furthercomprises a reboiler fluidly coupled to the distillation column, andwherein the step of controlling comprises adjusting at least oneoperational parameter of the reboiler in response to analytical dataproduced during the step of analyzing.
 5. An analysis method accordingto claim 4 wherein the step of adjusting at least one operationalparameter of the reboiler comprises adjusting the reboiler duty inresponse to analytical data produced during the step of analyzing.
 6. Ananalysis method according to claim 1 further comprising the step ofwithdrawing an extract stream and a raffinate stream from the extractionunit, and wherein the step of analyzing comprises measuring at least onequantity within the vapor sample indicative of raffinate impuritieswithin the extract stream.
 7. An analysis method according to claim 1further comprising the step of withdrawing an extract stream and araffinate stream from the extraction unit, and wherein the step ofanalyzing comprises measuring at least one quantity within the vaporsample indicative of extract impurities within the raffinate stream. 8.An analysis method according to claim 6 wherein the distillation columncontains a feed tray and a bottommost tray, and wherein the step ofdrawing a vapor sample comprises taking a vapor sample through a vapordraw fluidly coupled to the distillation column between the feed trayand the bottommost tray.
 9. An analysis method according to claim 1wherein the step of exposing comprises contacting a hydrocarbon feedstream with an aromatic-selective solvent within or upstream of thedistillation column, and wherein the step of drawing a vapor samplecomprises taking from the distillation column a vapor sample containingless than about 1 wt % of impurities of interest.
 10. An analysis methodaccording to claim 1 wherein the step of exposing comprises contacting ahydrocarbon feed stream with an aromatic-selective solvent within thedistillation column, and wherein the step of drawing a vapor samplecomprises taking from the distillation column a vapor sample containingless than about 0.5 wt % impurities of interest.
 11. An analysis methodaccording to claim 1 further comprising the step of returning thecondensed vapor sample or a portion of the condensed vapor sample to thedistillation column after analysis.
 12. An extraction unit forseparating a feed stream utilizing a species-selective solvent, theextraction unit comprising: a distillation column configured to receivethe feed stream and the species-selective solvent; a vapor draw fluidlycoupled to the distillation column and configured to draw vapor samplestherefrom; and an analyzer system fluidly coupled to the vapor draw andconfigured to measure the presence of at least one component within thevapor samples.
 13. An extraction unit according to claim 12 wherein theanalyzer system comprises: a condenser fluidly coupled to the vapor drawand configured to condense vapor samples received therefrom; and ananalyzer fluidly coupled to the condenser and configured to analyzecondensed vapor samples received therefrom.
 14. An extraction unitaccording to claim 13 wherein the analyzer comprises at least one of thegroup consisting of a gas chromatograph, a near-infrared spectroscopicinstrument, and a Fourier transform infrared spectroscopic instruments.15. An extraction unit according to claim 13 wherein the analyzer systemfurther comprises a phase separator fluidly coupled between thecondenser and the analyzer.
 16. An extraction unit according to claim 13wherein the analyzer system further comprises a controller operablycoupled to the analyzer and configured to adjust at least oneoperational parameter of the extraction unit in response to analyticaldata generated by the analyzer.
 17. An extraction unit according toclaim 16 further comprising a reboiler fluidly coupled to thedistillation column, the controller operably coupled to the reboiler andconfigured to adjust the heat duty thereof in response to analyticaldata generated by the analyzer.
 18. An extraction unit according toclaim 12 further comprising a vapor sample return conduit fluidlycoupling the analyzer system to the distillation column.
 19. Anextraction unit according to claim 12 further comprising a feed traydisposed within the distillation column, the vapor draw fluidly coupledto distillation column below the feed tray.
 20. An extraction unitaccording to claim 19 wherein the distillation column includes a loweroutlet through which a rich solvent stream flows, and wherein theanalyzer is configured to measure at least one quantity within the vaporsample indicative of raffinate impurities within the rich solventstream.